pps : add non blocking option to PPS_FETCH ioctl.

pps : add non blocking option to PPS_FETCH ioctl.

The PPS_FETCH ioctl is blocking still the reception of a PPS event.  But,
in some case, one may immediately need the last event date.  This patch
allow to get the result of PPS_FETCH if the device has the O_NONBLOCK flag
set.

Signed-off-by: Paul Chavent <paul.chavent@onera.fr>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Cc: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>